Consumer and commercial gas burning appliances, such as furnaces, hot water heaters, etc., combust gaseous fuel, e.g., natural gas, propane, etc., to generate heat to heat air or water. The ignition control systems for such gas burning appliances typically use a glow plug or hot surface igniter to ignite the gas released from a gas control valve into the combustion chamber. Such control systems typically include a flame sensor and circuitry that is utilized by the controller to detect the presence of flame and ensure safe operation of the gas burning appliance. That is, the controller will monitor the flame sense circuitry after the gas valve has been commanded open to ensure that ignition of the gas has occurred before an unsafe amount of gaseous fuel has been released through the gas control valve. This period of time varies by manufacturer, but may be within the range of four to seven seconds. If no ignition has occurred prior to the expiration of the timeout period, the gas control valve will be commanded closed, and the controller will allow for a purge time to expire before attempting to restart the burner.
Since the release and build up of un-ignited gaseous fuel presents a safety concern of explosion, typical ignition control systems drive the hot surface igniter with a voltage sufficient to generate a high enough temperature to ensure ignition of the released gaseous fuel. Many control systems allow for a short period of time to pass once the hot surface igniter has been energized to ensure that its surface temperature has achieved a sufficient temperature to ignite the gaseous fuel before opening the gas control valve.
The problems with many conventional ignition control systems, however, are two-fold. First, turning on energization to a hot surface igniter creates great thermal and mechanical stress in the device as the power is applied at a high voltage level and the temperature of the device rapidly increases to its maximum temperature. Such rapid heating and sustained high temperature operation results in premature failure of the hot surface igniter, greatly increasing the total cost of ownership of such appliances as well as decreasing customer satisfaction. Second, such conventional control of the hot surface igniter also increases the power consumption of the appliance, which makes conformance with government regulations regarding power consumption of appliances more and more difficult to meet.
To address such problems with prior hot surface igniter control systems, newer adaptive designs enabled by the use of microprocessor-based electronic controllers have been implemented. Typically, such more modern hot surface ignition control systems will adaptively reduce the energization voltage to the hot surface igniter upon subsequent ignition events to reduce the thermal stress and power consumption of the device. Such adaptation is required because the minimum ignition temperature may vary and depends on several factors such as burner configuration, gas pressure, igniter resistance variations, etc. Unfortunately, such systems have been plagued with a serious problem.
Specifically, prior adaptive controls operate to reduce the voltage applied to the hot surface igniter until the gas can no longer be ignited resulting in a release of un-burnt fuel, and then increase the voltage slightly. That is, upon an initial ignition event, the controller will turn on the energization to the hot surface igniter at a predetermined voltage level, typically its maximum voltage drive, for a short period of time to allow it to reach its maximum temperature, before commanding the gas control valve to open. The electronic controller will then monitor the flame sense circuitry to ensure that the gaseous fuel has been ignited within its ignition period. Upon a second ignition event, the controller will once again energize the hot surface igniter, but at a lower voltage level than the previous level. After the hot surface igniter has been given time to reach its maximum surface temperature at this new, lower, voltage level, the controller will open the gas control valve. The controller will once again monitor the flame sense circuitry to ensure that ignition of the gas occurs within the ignition period.
This process of reducing the drive voltage to the hot surface igniter continues until the gaseous fuel fails to ignite during the ignition period. Once this condition occurs, the ignition controller will allow a purge period to pass before again attempting to turn on the burner. Upon such an attempt, the electronic controller will energize the hot surface igniter at a voltage level greater than the voltage level in the previous ignition attempt during which the gaseous fuel failed to ignite. This process continues until the electronic controller has identified the minimum voltage necessary to drive the hot surface igniter that will ensure ignition of the gaseous fuel during the ignition period.
While such adaptive control is likely to extend the operating life of the hot surface igniter due to the lower drive voltage applied over most of the igniter's life, the adaptive control system itself does result in a release of gaseous fuel during the ignition period which will not be ignited during at least one of the ignition trials. This is because the controller reduces the drive voltage to the hot surface igniter below the point at which the gaseous fuel will ignite. This will result in at least one ignition attempt when gaseous fuel will be released for, typically, four to seven seconds without being ignited by the hot surface igniter. As discussed above, such failure to ignite conditions raise safety concerns, delay operation of the appliance for at least a purge period, and may result in a user believing the appliance has malfunctioned if the user smells the un-combusted gas that has been released for four to seven seconds. In such a situation, the user is liable to lose confidence in the appliance, believe the appliance is malfunctioning, and/or call for unnecessary service that will, as described above, increase the total cost of ownership and decrease the customer satisfaction with the appliance.
In view of the above, there is a need in the art for a hot surface igniter combustion control system that increases the life of the hot surface igniter and that does not result in the un-combusted release of gaseous fuel. Embodiments of the present invention provide for such adaptive control. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.